Interpretive Summary: Alfalfa leafcutting bees are the most common alfalfa pollinator in the Pacific Northwest. Reports from users of leafcutting bees in Idaho, Utah and Colorado have indicated exceptionally poor bee return from fields treated with an insect growth regulator, novaluron, to control Lygus bugs. Our goal was to evaluate novaluron toxicity to immature leafcutting bees using two different possible mechanisms of exposure. The objectives were to assess whether immature bees (eggs or larvae) die if nectar-pollen provisions and adults are contaminated with this pesticide. Immature bee death in all novaluron provision dosing treatments was significantly higher than the water or blank controls, providing evidence that novaluron is toxic to bee offspring in nest cells. More eggs and young larvae died compared to older larvae. Adult female bees nested similarly in field cages during the field cage experiment; however, there was greater offspring death in cages where females were fed sugar-water + novaluron compared to sugar-water only. Although females provided adequate provisions, there was a low percentage of egg hatch or larval development when females drank novaluron before mating and nesting. Novaluron could be contributing to poor bee return in alfalfa grown for seed. Timely insecticide applications to suppress Lygus bugs is an important consideration to improve ongoing bee health.

Technical Abstract:
Alfalfa leafcutting bees, Megachile rotundata (Hymenoptera: Megachilidae), are the most common alfalfa pollinator in the Pacific Northwest. Reports from users of M. rotundata in Idaho, Utah and Colorado have indicated exceptionally poor bee return from fields treated with novaluron to control Lygus spp. Our goal was to evaluate novaluron toxicity to immature M. rotundata using two different possible mechanisms of exposure. The objectives were to assess immature mortality via dosing nectar-pollen provisions and adults. Immature M. rotundata mortality in all novaluron provision dosing treatments was significantly higher than the water or blank controls, providing evidence that novaluron is toxic to progeny in nest cells. The mean cumulative frequency showed that more eggs and 1st-2nd instars died compared to older instars. Female M. rotundata nested similarly in field cages during the field cage experiment; however, there was greater immature mortality in cages where females were fed sugar-water + novaluron compared to sugar-water only. Although females provided adequate provisions, there was a low percentage of egg hatch or larval development when females ingested novaluron before mating and nesting. Novaluron could be contributing to poor bee return in alfalfa grown for seed. Timely insecticide applications to suppress Lygus spp. is an important consideration to improve ongoing bee health.